Changing constants in visual basic

I am putting together a little programme to calculate VAT. I don't want to enter the current rate every time, so I have declared its value to be a constant.
However, I would like to have an option to change it if the rate is changed. Is there any way of doing it from within the programme rather than having to change the code?
When I try to do so via assignment I get an error message!
Any suggestions gratefully received!